This did not work for me in latest Phaser, so I debugged through the code. Turns out I had to add prefix and suffix to config. (And no I didn't read the documentation )
My frames where called 'Idle (0).png" and so forth and when it checks for the texture name, it just goes from start to end as the name. So you have to add prefix and suffix too.
Like so:
this.anims.create({ key: 'girl_idle', frames: this.anims.generateFrameNames('girl', { start: 0, end: 16, prefix:'Idle (', suffix: ').png' }), repeat: -1 });